This invention relates to devices for providing drag or resistance to movement of mechanical members, such as in a telescoping strut, or a rod sliding within a surrounding sleeve or tube. The devices obtained the drag by the friction arising from the sliding of dry surfaces which is sometimes referred to as Coulomb Damping. The amount of drag or resistance to sliding movement is, of course, a function of the coefficient of friction of the engaging surfaces. Although such coefficient is often treated as a constant, it actually changes with temperature, humidity, wear of the engaging surfaces, as well as other operating conditions.
In many mechanical arrangements or systems, it is desirable that one member be slidable with respect to another with a known amount of drag resisting such movement. Such predictability is helpful in the design and use of the drag device and the system in which it is employed. In one specific example, it is desirable that a swingable member, such as a door or cover, be opened or closed only by overcoming a known drag. Many commercial airliners are provided with overhead storage compartments which are manually opened and closed. In an arrangement wherein the bin portion swings downwardly from a closed position to an open position, a control means is employed that resists the downward opening movement so that it must be manually moved, thus preventing the compartment from falling quickly once it has been released. One known device for this purpose employs a plurality of balls in an elastomeric tube to provide the resistance. Such an arrangement has the advantage of being relatively inexpensive, but it has the great disadvantage that it wears out fairly quickly and must be replaced, which results in costly labor expense. In a large commercial airliner, it is reportedly necessary to replace or repair one of the devices almost every day of operation. Such a device provides drag that changes with changes in friction. Also, it is desirable that a minimum of friction be introduced during the upward closing movement of the bin so that the user only has to overcome the weight of the bin.
If the storage compartment has a door that swings upwardly in an opening direction, the reverse operation would be desirable. That is, it is desirable that drag be minimized in the upward opening direction and that a constant drag be provided in the closing direction to prevent the door from inadvertently slamming shut. Similarly, such a strut might be employed for holding other doors open, such as a cowl providing access to an engine nacelle in an airplane or other vehicle. In high temperature applications, a drag device not subject to changes in the coefficient of friction is particularly advantageous. Also, air or hydraulic dampers are often not practical in such areas.
In other situations, it may be necessary to maintain a constant resistance to the movement of one member relative to another in either of two opposite directions. In one such example, it is desirable that an aircraft control stick have a certain resistance to movement or "feel". Typically, this resistance to movement comes from the control cables and/or the control elements moved by the cables. However, in so-called "fly-by-wire" control systems that are primarily electronic, the pilot may only be moving an electrical potentimeter in moving the control stick. Since the moving of an electrical potentimeter provides no significant mechanical load, it is desired that an "artificial feel" be provided on the control cable or other element. It is important that this drag be substantially constant so that the control will always feel the same to the pilot.
These are but a few examples that illustrate that a need exists for simple but reliable devices which will provide a substantially constant frictional drag.